The crane-ladle is carried from the furnace to the mould by the swinging and traversing motions of the crane, which is similar to those used at the iron forges, etc. (see p. 196,) and in very the air. is insoluble, and serves partly as a defence to the metal beneath. When first raised from the tea-water, the plumbago becomes exceedingly hot from the action of the atmosphere; it may be cut with a knife like an ordinary pencil. - Minutes of Conversation, Inst. Civ. Engineers, 8th Feb. 1842
* The furnace is not unfrequently tapped whilst the charge of metal is being melted, and in such cases when the required quantity has been removed into the ladles, the fireman re-stops the tap-hole, by a conical plug of clay on the end of a wooden bar; the process is called hotting, and requires a dexterous hand, or the whole contents of the furnace may escape.
The bail, or handle of the crane-ladle, is fixed in its perpendicular position by the guard, a simple bolt which prevents the ladle from being overset by accident until it has reached its destination. Two long handles, terminating in forked branches, are now fitted by their square sockets upon the swivels or pivots of the crane-ladle, and secured by transverse keys, after which the guard is withdrawn; and then two men at the ladle, two others at the crane, and one to skim the dross from the lip of the ladle, commonly suffice to manage two or three tons and upwards of fluid iron, with great ease and dexterity.*
The observations offered on p. 344, respecting the temperature of the metal suitable to different brass works, might be here in a great measure repeated: namely that the smallest castings require very hot metal, and a gradually lower temperature is more suitable to works progressively heavier, to avoid their becoming sand-burned or rough on the face, from the partial destruction of the mould.
When cast-iron is very hot, the metal scintillates most beautifully, far more vividly than a mass of wrought-iron raised above the welding heat; as the metal cools, the sparks become intermittent, and at last the metal remains entirely quiet, excepting a multitude of lines vibrating in all directions, as if the surface were covered with thousands of wire-worms in great activity; this effect lessens until the metal solidifies. The softest iron shows most of this play of lines, or is said to break the best.
Iron castings are generally much heavier than those of brass, and the melting heat of the metal being considerably higher, the quantity of gas generated is very much greater; additional care is consequently required to provide for its escape, or the explosions are much more violent. The sand is punctured at many places with a fine wire, before the removal of the patterns; sometimes also more coarsely as soon as the metal has become solidified. The gases issuing from the filled moulds are often lighted, either by the red-hot skimmer, or by a torch of straw with which the moulds are flogged: this lessens the accumulation of gas and the consequent risk of accident.
* Mr. Nasmyth has added to the pivot of the large crane-ladle, a tangent-screw and worm-wheel, by which it may be gradually tilted by one man standing directly in front at any convenient distance; and another man skims the metal by a kind of throttle-valve coated with clay, which sweeps into the lip of the ladle, and keeps back the sullage: the axis of the skimmer is continued as a long rod, at right angles to the first, and also terminating in a cross. By these arrangements any precise quantity of metal can be delivered, and the risk of accident scarcely exists.
The pouring of very large objects in open moulds, such as plates, beams and girders, is a very beautiful and grand sight. The metal is led from the furnace, through a gutter lined with sand, into; a large trough or tow, the end of which is closed with a shuttle; when the sow is full, the shuttle is raised; this allows the metal to flow very quickly into the mould, but enables it to be kept back should it be unnecessarily hot; the castings made in open moulds are generally covered up with sand as soon as the metal is set.
The above, and the casting of smaller objects, such as flat plates in open moulds, may appear amongst the most certain modes of procuring sound castings; but unless the air be well drawn from the lower surfaces, they will become honey-combed or full of air-bubbles. This defect is avoided by making the sand-bed sufficiently porous, and pricking it with many holes just below the surface, to serve as horizontal air-drains.*
* In casting lead, tin, etc. on a flat metallic plate, the formation of air-bubbles is lessened by placing a sheet of dry paper on the plate; it appears to keep down any little babbles of sir or vapour, and to provide a thin channel for their escape.
But the most perfect example of a porous mould is that invented by Lord Oxmantown, to avoid the formation of air bubbles in those speculums which are cast in open moulds. The plate, or bed of the mould, consists of a great number of (dips of hoop-iron placed edgeways and in contact; they are screwed tight within a frame, and are then turned in a lathe to the required curve: by this arrangement iuterstices exist at nearly every point through which the air may escape downwards.
Speculum metal is perhaps the most untractable of any of the alloys, and it serves to illustrate in a most striking manner many of the effects that occur in castings generally. Small speculums are cast in sand; as soon as they are set, the sand core is pushed out of the aperture in such as are intended for Gregorian telescopes, to enable them to contract without fracture, and the red-hot disk is surrounded by ignited wood-ashes or any very bad conductor, to delay the cooling
These precautions entirely fail with large speculums, as the margins solidify the first, and from the absence of ductility, the central parts tear away in the act of contraction, and the mass becomes rent or flawed. Lord Oxmantown considered this fracture would be avoided by cooling the speculums in uniform layers from below upwards, or as it were in infinitely thin laminae, and he therefore first employed iron moulds which were cooled by a stream of water projected against their under